"It's going to rain this afternoon" 是表示 "今天下午要下雨" 的一种常用方式。其中 "It's going to" 是一个预测性的短语，表示 "将要" 的意思，而 "rain" 是 "下雨" 的意思。
例句：
"It's going to rain this afternoon, so make sure to bring an umbrella." (今天下午要下雨，所以一定要带伞。)
"I heard that it's going to rain this afternoon. Should we reschedule our picnic?" (我听说今天下午要下雨。我们是否应该改期野餐?)
"It's going to be a rainy afternoon, so I'm going to stay inside and watch movies." (今天下午将会是下雨的，所以我打算待在家里看电影。)
此外，还有一些表达方式如 It's supposed to rain this afternoon. It's forecasted to rain this afternoon.